1. Field of the Invention
The present invention relates to a powder magnetic core used with various electric and electronic devices.
2. Prior Art
Recently, there is a growing requirement, in the construction of very compact electric and electronic devices, for very compact, greatly efficient powder magnetic cores. Powder magnetic cores fabricated by the compression of iron base ferromagnetic metal powders have large saturation magnetizations and so are favorable for size reductions. Sendust (Fe-Al-Si alloy) powder magnetic cores are lower in material cost than molybdenum permalloy (Fe-Ni-Mo alloy) powder magnetic cores, but they are in no sense superior to the permalloy cores in terms of permeability and power losses. Difficulty is involved in reducing the size of sendust cores used with choke coils or inductors, because large core losses result in some considerable core temperature rise. For instance, when a certain sendust powder magnetic core is built in a power supply portion of an inductor in a power-factor improving circuit, the core loss at 100 kHz and 100 mT, for example, must be reduced to preferably 450 kW/m.sup.3 or less, more preferably 300 kW/m.sup.3 or less.
For instance, some proposals have been made of loss reductions for sendust powder magnetic cores, as mentioned just below.
JP-B 62(1987)-21041 alleges that an iron-silicon-aluminum base magnetic alloy powder magnetic core higher in permeability and yet lower in core losses than molybdenum permalloy cores is obtainable by annealing an iron-silicon-aluminum base magnetic alloy ingot at 700.degree. to 1,100.degree. C., then pulverizing and pressing the annealed product, and finally firing the powder compact at 600.degree. to 800.degree. C. in a hydrogen atmosphere. One example in this publication shows that a powder magnetic core having a permeability of 146 at 10 kHz and core losses as measured at 25 kHz of 158 kW/m.sup.3 at 1,000G and 548 kHz/m.sup.3 at 2,000G is obtained by regulating the powders to 32 meshes or less, pressing them, and firing the pressed compact at 700.degree. C.
For an inductor used with power-factor improving or other circuits, however, it is still desired to achieve further core loss reductions.
In view of the problem as above described, an object of the present invention is to provide a powder magnetic core having low core losses at low costs. Another object of the present invention is to provide a powder magnetic core having low core losses, and high mechanical strength as well.